Re: Postgres on RAID5

I would recommend running a bonnie++ benchmark on your array to see if
it's the array/controller/raid being crap, or wether it's postgres. I
have had some very surprising results from arrays that theoretically
should be fast, but turned out to be very slow.

I would also seriously have to recommend against a 14 drive RAID 5!
This is statisticaly as likely to fail as a 7 drive RAID 0 (not
counting the spare, but rebuiling a spare is very hard on existing
drives).

Alex Turner
netEconomist

On Fri, 11 Mar 2005 16:13:05 -0500, Arshavir Grigorian <ag(at)m-cam(dot)com> wrote:
> Hi,
>
> I have a RAID5 array (mdadm) with 14 disks + 1 spare. This partition has
> an Ext3 filesystem which is used by Postgres. Currently we are loading a
> 50G database on this server from a Postgres dump (copy, not insert) and
> are experiencing very slow write performance (35 records per second).
>
> Top shows that the Postgres process (postmaster) is being constantly put
> into D state for extended periods of time (2-3 seconds) which I assume
> is because it's waiting for disk io. I have just started gathering
> system statistics and here is what sar -b shows: (this is while the db
> is being loaded - pg_restore)
>
> tps rtps wtps bread/s bwrtn/s
> 01:35:01 PM 275.77 76.12 199.66 709.59 2315.23
> 01:45:01 PM 287.25 75.56 211.69 706.52 2413.06
> 01:55:01 PM 281.73 76.35 205.37 711.84 2389.86
>
[snip]